Hitherto, automatic feed of a work into and out of an automatic press system was conducted by means of a work feeder (abbreviated as "feeder", hereafter) which operates in accordance with a predetermined motion curve by being triggered by, for example, a signal from a limit switch which operates when a predetermined angle has been reached by the crank of the press machine. The feeder operates so as to feed the work into and out of the press machine so as to follow a predetermined motion curve, by a specific construction of a link mechanism and a suitable control of the operation speed of a motor. The arrival of the feeder at a predetermined position is detected by, for example, a limit switch and a signal output from the limit switch is transmitted to the press machine, thereby causing the press machine to initiate the next cycle of operation.
In another known method, shafts of a feeder are controlled by a specific combination of cams operatively associated with the crank of the press machine or by other mechanical action linked with the crank.
The method which relies upon a limit switch suffers from a disadvantage in that the next cycle of operation has to be commenced before confirming completion of the instant cycle of operation, in view of a time tolerance or variation in the mechanical operation triggered by the signal from the limit switch. On the other hand, the method relying upon mechanical means is disadvantageous in that there is a practical limit in the operation speed. Therefore, in order to realize a higher speed of operation of the press machine system with higher degree of reliability, it is necessary to obtain a closer correlation between the feeder operation and the crank angle of the press machine. Under this circumstance, in recent years, it has become popular to use, for example, a servo mechanism in which the feeder position is electrically controlled with high precision in accordance with the crank angle which is measured by a synchronous measuring device.
A description will now be given of a circuit for effecting the above-described control, with specific reference to FIG. 9. Referring to FIG. 9, numeral 82 designates a synchronous transmitter which is attached to the crank shaft of a press machine and which can measure the absolute value of the angle of rotation of the crank, while 83 denotes a conversion circuit which converts the angle information derived from the synchronous transmitter into digital codes. The digital codes indicative of the crank angle of the press machine are delivered to a computer 81. The computer 81 reads position information for each of a feed shaft, a lift shaft and a clamp shaft of the press machine, the information being beforehand determined in relation to the crank angle and stored in a memory device. The computer 81 then forms a position command signal in accordance with the read information. Numeral 88 collectively designates position detecting synchronous transmitters which are attached to respective operational shafts. In FIG. 9, the construction is shown in regard to only one shaft, because the constructions for the other two shafts are similar. Position information measured by the synchronous transmitter 88 attached to, for example, the feed shaft, is converted into digital code by means of a conversion circuit 89 and is inputted to a controlling computer 81. The feed shaft position information thus inputted to the computer is compared with a previously determined position command signal and the computer 81 forms an offset signal in accordance with the result of the comparison. Subsequently, a speed command signal for a feed shaft drive motor, which is beforehand stored in the memory device and which corresponds to the level of the offset signal, is read and outputted after being coded into a digital code. The speed command signal for the feed shaft drive motor is converted into an analog signal by an analog conversion circuit 84 and is amplified by a servo amplifier 85 to an appropriate level of power by which the servo motor 86 is driven. The servo motor 86 drives the feed shaft. Meanwhile, a tacho-generator 87 mechanically coupled to the servo motor 86 measures the number of rotations of the servo motor shaft, the result being returned to the servo amplifier 85. Thus, a feedback loop is formed to enable a stable control of the rotation of the motor. The synchronous transmitter 88 for detecting the aforementioned feed shaft position information is connected to the feed shaft, so as to input correct data of the instant position of the feed shaft. It is therefore possible to control the feeder in accordance with the crank angle of the press machine such as to meet a predetermined condition.
In this type of system, the operation of the feeder is fixed in relation to the crank angle of the press machine. Namely, only a fixed operation angle can be obtained for a given crank angle, so that the motion pattern is fixed. (It is to be noted, however, that the operation strokes of the respective shafts of the feeder are semi-fixed in the case of a pure mechanical driving system and are variable in the case of an electrical synchronous type system.) The fact that the operation angles are fixed poses a restriction in the design of the press dies and makes it impossible to fully use the performance of the motors. More specifically, in consideration of the risk for interference with the die, the operation angles of the respective shafts are preferably set ahead during upward stroking of the press machine and set back during downward stroking of the press, in order to reduce loads on the respective motors and to facilitate the control, while reducing the production cost. The possibility of free setting of the operation angles and strokes of the respective shafts in accordance with the size and configuration of the die advantageously reduces restriction in the design of the die and enables the performance of the motors to be fully utilized.
On the other hand, the ability to vary the operation angles in accordance with the die specifications causes the specifications of the drive motors of the press machine to be exceeded. For instance, a too small pitch of the operation angle may cause the drive motor to operate at a speed higher that the maximum allowable speed. Similarly, an acceleration torque exceeding the instantaneous maximum allowable torque may be required. This leads to various problems. For instance, if the press machine operates at a press crank rotation speed which is determined in accordance with set operation angle data or set shaft stroke data, the position of the feeder may become out of phase with the press crank angle, due to insufficiency of the performance of the motors of the feeder, with the result that the system is emergency-stopped due to the mismatching of phase between the press machine and the feeder or the motor stops operation due to overload. It takes an impractically long time for the detection of the cause of the stopping of the press system.
In view of the two problems described above, an object of the present invention is to provide a work feeder device which is capable of automatically setting optimum motion curve in accordance with the specifications of the press die, so as to enable full use of performance of the motor for driving each shaft of the feeder while averting from interference with the die, and which can detect any undue operating conditions thereby preventing accidental stopping of the motor operation which may otherwise be caused by such undue operating conditions.